A continuously variable transmission (CVT) is a type of power transmission that is capable of achieving infinite variability between a highest and a lowest possible speed ratio. Unlike conventionally-geared transmissions that use one or more planetary gear sets and multiple rotating and braking friction clutches to establish a discrete gear state, a CVT uses a variable-diameter pulley system known as a variator assembly to transition anywhere within a calibrated range of speed ratios.
A typical variator assembly includes two variator pulleys interconnected via an endless rotatable drive element such as a chain or a belt. The drive element rides in a variable-width gap defined by conical faces of mating halves of the pulleys. One of the pulleys is typically connected to an engine crankshaft and thus acts as a driving/primary pulley. The other pulley is connected to a CVT output shaft to act as a driven/secondary pulley. One or more gear sets could be used on the input and/or output side of the variator assembly depending on the design. In order to vary a CVT speed ratio, a shift force is applied to the primary pulley and/or the secondary pulley via one or more pulley actuators. The shift force effectively squeezes the pulley halves together to change the width of the gap between the conical pulley faces. Variation in the gap size, which is also referred to as the pitch radius, causes the drive element to ride higher or lower within the gap. This in turn changes the effective diameters of the variator pulleys and thus the speed ratio of the CVT.